Pumice is born in pyroclasic events, typically spewed as ash that falls and drifts into deposits or as part of a massive pyroclastic flow. Many factors affect the useful quality, color, and purity of a pumice deposit, but on a chemical analysis level, pumice is basically an aluminum silicate. With no crystalline structure, pumice is a naturally calcined amorphous glass made up of a maze of air-filled vesicles. It can absorb water and nutrients quite well. When used in a Bonsai soil mix, it helps to retain water and it aids the roots to ramify very well.